In recent years, an increase in size of a wafer is demanded, and a wire saw is mainly used to slice a workpiece with this increase in size.
The wire saw is an apparatus that causes a wire (a high-tensile steel wire) to travel at a high speed, presses a workpiece against the wire to cut the workpiece while a slurry is applied to the wire and thereby slices the workpiece into many wafers at the same time.
Here, an outline of an example of a conventionally general wire saw is shown in FIG. 3.
As shown in FIG. 3, a wire saw 101 mainly includes a wire 102 for slicing a workpiece, grooved rollers 103 around which the wire 102 is wound, a mechanism 104 for giving the wire 102 a tensile force, a mechanism 105 for feeding the workpiece to be sliced downward, and a mechanism 106 for supplying a slurry at the time of slicing.
The wire 102 is unreeled from one wire reel 107 and reaches the grooved rollers 103 via the tensile-force-giving mechanism 104 composed of a powder clutch (a constant torque motor 109), a dancer roller (a dead weight) (not shown) and the like through a traverser 108. The wire 102 is wound around this grooved rollers 103 for approximately 300 to 400 turns to form a wire row, and then taken up by a wire reel 107′ via the other tensile-force-giving mechanism 104′.
Moreover, each of the grooved rollers 103 is a roller that has a steel cylinder of which a polyurethane resin is pressed in the periphery and that has grooves formed at a fixed pitch on a surface thereof. The wound wire 102 can be driven in a reciprocating direction for a predetermined traveling distance by a driving motor 110.
It is to be noted that the workpiece-feeding mechanism 105 feeds the workpiece toward the wire 102 wound around the grooved rollers 103 by holding and pushing down the workpiece at the time of slicing the workpiece.
Moreover, nozzles 115 are provided near the grooved rollers 103 and the wound wire 102, and the slurry can be supplied to the wire 102 from a slurry tank 116 at the time of slicing. Additionally, a slurry chiller 117 is connected with the slurry tank 116 so that a temperature of the slurry to be supplied can be adjusted.
With the wire saw 101, an appropriate tensile force is applied to the wire 102 with a wire-tensile-force-giving mechanism 104, the wire 102 is caused to travel in a reciprocating direction with the driving motor 110 and free-abrasive grains in the supplied slurry are pushed into an inner portion of a wire groove (a slicing groove of the workpiece) by the wire 102 to scrape a bottom portion of the groove of the workpiece so that the workpiece is sliced.
Friction that occurs in this case causes generation of heat at a slicing portion of the workpiece and thereby the workpiece is thermally expanded during slicing. Moreover, the grooved roller is axially expanded by thermal expansion under frictional heat between the wire and the grooved roller and under frictional heat generated at a bearing portion, which sustains the grooved roller. Consequently, there are instances that a relative position between the workpiece and the wire row, which is spirally wound around the grooved rollers, changes during slicing.
It has been known that displacement of the grooved roller in an axial direction has a constant correlation with a temperature of the slurry supplied during slicing the workpiece (See Japanese Patent Laid-open (Kokai) No. H5-185419).
A change in the relative position between the workpiece and the wire row, which is caused by the thermal expansion of the workpiece and the expansion of the grooved roller in an axial direction, brings about inflecting slicing trajectory that is depicted in the workpiece by the wire 102. There is a problem that this inflection of the slicing trajectory is detected as Warp in a shape measurement after wafer processing.
For solving the problem regarding the generation of heat in the workpiece during slicing, there is disclosed a wire saw that has a workpiece cooling means provided in a workpiece holding member and slices the workpiece with cooling it (See Japanese Patent Laid-open (Kokai) No. H11-58365).
However, in a method for slicing the workpiece by using this wire saw, it is difficult to have a sufficient cooling capability since a slicing portion is far from a cooling portion and a size of the cooling portion is restricted, even if the workpiece is cooled with the workpiece cooling means provided in the workpiece holding member during slicing. Additionally, this method cannot control the thermal expansion of the grooved roller.
On the other hand, regarding the thermal expansion of the grooved roller during slicing, there is disclosed a wire saw that has bearings for rotatably sustaining a plurality of the grooved rollers, a cooling-medium-supplying means for supplying a cooling medium that cools each of the bearings and a cooling-medium-temperature-adjusting means for adjusting a temperature of the cooling medium (See Japanese Patent Laid-open (Kokai) No. 2003-145406).
Although this wire saw can adjust the temperature of the cooling medium to be supplied to an inner race side of the bearings, which sustain the grooved rollers, it is difficult for the wire saw to cool an outer race side of the bearings. Since the thermal expansion of the grooved roller is also affected strongly by a temperature of the slurry supplied to a peripheral surface of the grooved roller, it is difficult to effectively reduce the thermal expansion of the grooved roller by the method of adjusting only the temperature of the cooling medium supplied to a shaft part of the grooved roller. Additionally, this wire saw cannot control the thermal expansion of the workpiece during slicing.
Moreover, there is disclosed a method for slicing the workpiece while a change in temperature of the workpiece is made small, for example, by supplying the slurry having a constant temperature during slicing the workpiece or the slurry having a predetermined temperature by controlling the temperature in Japanese Patent Laid-open (Kokai) No. 2003-1624.
Moreover, there is disclosed a wire saw that has a means for pouring the slurry of which a temperature is controlled on the workpiece directly or a means for jetting a medium of which a temperature is controlled on the workpiece directly to control the temperature of the workpiece in International Publication Pamphlet WO00/43162.
However, any of the foregoing conventional arts have such problems that a special mechanism needs to be added to the wire saw or an improvement effect on the Warp is insufficient.